Much room for improvement.
Graphics:
Very simple, but a few things like red footprints caused by stepping over blood and the overlapping red sights of the guards are nice add-ons. More detail for the characters would be appreciated though.
Sound:
The regular background music is not very encouraging and is rather annoying after a while. The alarm track doesn't serve its purpose either. Something more energetic or dangerous would be perfect in this situation.
A.I. (Artificial Intelligence):
It looked as if the guards just randomly walked around. Well, they are blue so they must be boozed *lol reference*. They should rather follow set paths instead of randomly changing the direction when they walk against an object or just because they seem to feel like it. Getting to the retreat point would be a lot easier this way, not to mention maneuvering in general.
Gameplay:
Easy on the surface, hard when you get to the levels above 12. Invisibility is a good device to avoid being seen but is useless if one or multiple guards happen to slowly walk at you. This is the moment when you have to run. Speed-up makes sense, but you should be slightly faster to better avoid bullets, run away, or run past guards unseen. By the way, if you can kill the green man, why can't you even scratch the guards? A stab in the back of the neck or something should be possible to do, even if they wear full body armor. Maybe this would make the game too easy but you could include achievements in the sequel to even it out. Something like "kill all guards in each level" or "kill a guard after he noticed you and still complete the level".
Overall:
Challenging game with a few flaws. A few improvements here and there and you are bound to receive countless of 10/10 ratings. On a side note, a level editor would be a nice bonus.